1. Field of the Invention
The present invention relates to electric heating devices and in particular, electric arc heaters, also known as plasma heaters.
2. Description of the Prior Art
Electric arc or plasma heaters are used in industrial processes requiring the heating of process gas or materials to high temperatures. The high temperature product exiting the heater has many useful applications in the reduction of ores and in various other chemical processes. The electric arc heater can be electrically powered from either an AC source or a DC source and have electrical power level requirements typically exceeding 1000 kilowatts. In operation, an electric arc is generated between two electrodes in the electric arc heater. The gas or material to be heated is injected into the arc heater where it contacts the electric arc and is heated to an extremely high temperature.
Present electric arc heater designs incorporate at least two cylindrical electrodes which are axially spaced apart to form an annular arc gap across which the electric arc is formed. Rings constructed of insulating materials are positioned between the two electrodes and are used to maintain the arc gap. The electrodes are connected to a source of electric potential of sufficient magnitude to start and maintain the electric arc. The material to be heated, usually a gas, is injected into the arc gap causing the electric arc to lengthen into the arcing chamber which is an area defined by the interior surfaces of the electrodes. The intense heat contained in the electric arc is transferred to the material as it passes through the arcing chamber. The heated material is the exhausted from the electric arc heater into a reaction vessel wherein the arc heated material undergoes further reaction or heats or reacts with other material present in the reaction vessel.
A further refinement of the electric arc heater provides for the addition of field coils surrounding each electrode. These field coils are connected to a source of electrical potential to establish a magnetic field which interacts with the current flowing in the electric arc according to Lenz's Law causing the electric arc to rotate about the interior surfaces of the electrodes. Electric arc rotation decreases the rate of electrode erosion and allows for better temperature distribution in the electrode. In addition, channels are provided in the electrode assembly and field coils for water-cooling of the electric arc heater.
Examples of electric arc heaters employing these various elements can be found in U.S. Pat. No. 3,705,975 entitled "Self Stabilizing Arc Heater Apparatus" issued Dec. 12, 1972, U.S. Pat. No. 4,042,802 entitled "Three Phase Arc Heater" issued Aug. 16, 1977, U.S. Pat. No. 4,214,736 entitled "Arc Heater Melting System" issued July 29, 1980 and U.S. Pat. No. 4,219,726 entitled "Arc Heater Construction With Total Alternating Current Usage" issued Aug. 26, 1980. These patents are assigned to the assignee of the present invention and are examples of electric arc heaters that can be modified to use the present invention.
Although the electric arc heater can be used as an independent device, it is typically mounted on a reaction vessel such as a furnace, cupola or kiln. One mounting means used is a flange about the outlet of the electric arc furnace. A corresponding mating flange against which this outlet flange abuts is provided on the reaction vessel. The two flanges are secured together by bolts or other conventional fastening means. In lieu of the mating flange, the outlet flange of the electric arc heater can be attached directly to the surface of the reaction vessel by conventional means.
With present designs, when the downstream addition of another material to the arc heated material is desired, a cylindrical admission collar is positioned intermediate the outlet of the electric arc heater and the reaction vessel. The additional material enters the flow path of the arc-heated material exiting the outlet via passageways provided in the collar wall. Because of the high temperatures present, the admission collar is either refractory lined or is water-cooled, and is a separate part requiring special design and handling, separate, assembly and maintenance, and usually separate water cooling means. Additional sealing must also be provided to prevent leaks into or from the reactor. Accordingly, it would be advantageous to provide for downstream admission of additional material into the arc-heated material without the use of separate admission collars and seals.
When the electric arc heater is attached to the reaction vessel, it has been found that the surfaces of the passageways through which the arc-heated material is conducted into the reaction vessel suffer thermal and erosive damage from their contact with the hot materials. Depending upon the nature of the hot materials passing from the electric arc heater, it has also been found that undesirable materials can accumulate on these surfaces downstream of the outlet of the electric arc heater. Therefore, it would be desirable to have a means for preventing or ameliorating the erosive damage caused by the hot materials exiting the arc heater, as well as preventing buildup of undesirable material on the surfaces of the passages downstream of the electric arc heater.